The present invention relates to filters for the removal of particulate material from diesel engine exhaust streams, and more particularly to a porous ceramic diesel exhaust filter of improved resistance to melting and thermal shock damage suitable for diesel exhaust systems which require long, narrow packaging.
Diesel traps have proven to be extremely efficient at removing carbon soot from the exhaust of diesel engines. The most widely used diesel trap is the wall-flow filter which filters the diesel exhaust by capturing the soot on the porous walls of the filter body. The wall-flow filter is designed to provide for nearly complete filtration of soot without significantly hindering the exhaust flow.
As the layer of soot collects on the surfaces of the inlet channels of the filter, the lower permeability of the soot layer causes a pressure drop across the filter and a gradual rise in the back pressure of the filter against the engine, causing the engine to work harder, thus affecting engine operating efficiency. Eventually, the pressure drop becomes unacceptable and regeneration of the filter becomes necessary. In conventional systems, the regeneration process involves heating the filter to initiate combustion of the carbon soot. Normally, the regeneration is accomplished under controlled conditions of engine management whereby a slow burn is initiated and lasts a number of minutes, during which the temperature in the filter rises from about 400-600xc2x0 C. to a maximum of about 800-1000xc2x0 C.
The highest temperatures during regeneration tend to occur near the outlet end of the filter due to the cumulative effects of the wave of soot combustion that progresses from the frontal inlet end to the outlet end as the exhaust flow carries the combustion heat down the filter. Under certain circumstances, a so-called xe2x80x9cuncontrolled regenerationxe2x80x9d can occur when the onset of combustion coincides with, or is immediately followed by, high oxygen content and low flow rates in the exhaust gas (such as engine idling conditions). During an uncontrolled regeneration, the combustion of the soot may produce temperature spikes within the filter which can thermally shock and crack, or even melt, the filter.
Even more excessive temperature spikes can occur in long filters (i.e., filters having a length greater than 20.32 cm (8 inches)); additionally, long filters experience a higher pressure drop, increased occurrence of radial or ring-off breaks due to thermal gradients and are also difficult to manufacture and package. Therefore, long filters are not preferred in the industry of emissions control, nonetheless, a long, narrow space is often the allotted space in vehicle for the emission control system.
It would be considered an advancement in the art to obtain a filter which is suitable for diesel exhaust systems which require long, narrow packaging and also survives the numerous controlled regenerations over its lifetime, and the much less frequent but more severe uncontrolled regenerations. This survival includes not only that the diesel particulate filter remains intact and continues to filter, but that the back pressure against the engine remains low.
The present invention provides porous ceramic particulate exhaust filters offering improved configurations, particularly for long length emission systems, that are more resistant to thermal cracking and melting damage under typical diesel exhaust conditions than current filter designs. At the same time, the filters of the invention offer significantly lower pressure drops across the filter and superior resistance to soot-induced back-pressure buildup, than designs currently available.
In particular, the invention provides a ceramic filter which includes a plurality of end-plugged honeycomb structures which in combination act to trap and combust diesel exhaust particulates. The inventive filters are configured to allow for substantially equal exhaust gas flow through each end-plugged honeycomb structure, such that a long filter configuration is possible without the drawback associated with long filter lengths.
Temperature spikes which occur during regeneration, and especially during uncontrolled regeneration, are reduced in the inventive filters. At the same time, the inventive filter design provides filters with a low initial filter pressure drop and a reduction in the system pressure drop during use.
The inventive filters are especially advantageous for diesel exhaust systems where vehicle underbody space is limited and typically confined to long regions. Inventive filters are particularly suitable for lengths greater than 20.32 cm (8 inches).
In one embodiment the diesel exhaust particulate filter includes an enclosure provided with an inlet cone and an outlet cone for channeling exhaust gas through the filter; a first end-plugged honeycomb structure positioned adjacent the inlet cone, and a second end-plugged honeycomb structure positioned adjacent the outlet cone, the first end-plugged honeycomb structure and the second end-plugged honeycomb structure being positioned off-center within the enclosure such that the honeycomb structures are parallel to the flow of exhaust gas; and, means, for separating the unfiltered exhaust gas entering the filter from the exhaust gas filtered by the honeycomb structures.
In another embodiment the diesel exhaust particulate filter includes an enclosure provided with an inlet cone and an outlet cone for channeling exhaust gas through the filter; a first end-plugged honeycomb structure positioned adjacent the inlet cone, and a second end-plugged honeycomb structure positioned adjacent the outlet cone, the first end-plugged honeycomb structure and the second end-plugged honeycomb being centrally positioned and mounted side-by-side such that the honeycomb structures are perpendicular to the flow of exhaust gas; and, means for separating the unfiltered exhaust gas entering the filter from the exhaust gas filtered by the honeycomb structures.